Why would SQL queries that use reserved words suddenly start causing problems?

In the past couple of days we have experienced issues with 3 different applications that have been running fine for several years. Errors #1 and #2 were caused by reserved words in SQL queries. I am fairly certain that is also the cause of Error #3, but I cannot be sure as I don't have the source code. The databases for the three applications are all on the same instance of SQL Server. We are running SQL Server 2008 with no updates, and the last Windows update was in 2016. What could have caused these errors to suddenly start occurring? Error details: 1. We have a Web Forms site that has been used daily (and not updated) for the past 6 years. Two days ago it suddenly started throwing a SQL timeout exception on one of its pages. In SSMS I tested the raw queries being used and they all ran in under 2 seconds. However I noticed that two of them used the reserved words "Status" and "Date" without square brackets. I added square brackets around the reserved words and re-uploaded the site, and that fixed the error. 2. Today users reported an issue with a report in an old MS Access Data Project. I tested the report myself and got a query timeout. The data source for the report is a table-valued function, which when I executed directly in SSMS completed in a matter of seconds. I examined the function body and saw that it used the reserved words "Priority" and "Timestamp". I added square brackets, altered the function, and voila, the report now opens with no errors. 3. We have an .asmx web service that has been used daily for the past 6 years. One of the methods is supposed to query a table and return a list of user logins and passwords. Yesterday it started returning an empty response object. I don't have the source code for the web service so I can't see the text of the query. However I can see that the table has columns named with the reserved words "User_ID" and "Password", and based on the pattern for errors #1 and #2 I would bet that that's the problem. These queries have been running for years without problems. Why would they suddenly start causing problems? Is there perhaps a configuration in SQL Server that has been changed?